Polyester multifilament yarns have found widespread use in various applications, and with increasing demands on mechanical performance of such fibers various high-strength polyester yarns have been developed with, among other improved parameters, relatively high modulus and relatively low free shrinkage.
For example, Nelson et al describe in U.S. Pat. Nos. 5,067,538 and 5,234,764 methods and compositions for a polyester multifilament yarn having a dimensional stability of E4.5+FS of less than 11.5% and a terminal modulus of above about 20 g/d. Among other desirable qualities, Nelson's yarns can typically be employed in environments with relatively high temperatures (here: 80-120° C.). Furthermore, crystallization of the poly(ethylene terephthalate) (PET) in Nelson's yarns appears to occur during spinning, thereby potentially rendering at least some of the desired mechanical qualities of the yarn independent from fluctuations during drawing.
In another example, Rim et al. describe in U.S. Pat. No. 5,397,527 methods for producing a multifilament yarn fabricated from poly(ethylene naphthalate) (PEN) or other semi-crystalline polyester having a dimensional stability (EASL+Shrinkage) of less than 5% and a tenacity of at least 6.5 g/d. Rim's yarns advantageously improve several mechanical qualities of previously known PEN yarns and may even be produced using equipment without high-speed spinning capability. However, in order to achieve most of the improvements in mechanical quality, the chemical composition of such yarns is typically limited to PEN or compositions with high quantities of PEN.
In a further example, U.S. Pat. No. 5,238,740 to Simons et al a polyester yarn with a tenacity of at least 10 g/d and a shrinkage of less than 8% is produced by passing the spun filaments through a heated and insulated column in which a particular temperature profile is employed in combination with relatively high take-up speeds to obtain the desired improved mechanical properties. While Simons' methods generally produce yarns with a relatively high tenacity and a relatively high secant modulus (greater than 150 g/d/100%) at a comparably low shrinkage, relatively expensive equipment and additional process controls for the heated column are generally required.
Although various compositions and methods for production of dimensionally stable yarns are known in the art, all or almost all of them require moderate to high cord twist for use in demanding fatigue applications such as tires. While global requirements for fatigue resistance have become increasingly stringent, there has not been the commensurate improvement in fatigue resistance to avoid the need for higher twist in the most demanding applications. There have been various approaches to improve fatigue resistance in dimensionally stable yarns (see e.g., U.S. Pat. No. 4,101,525 to Davis, U.S. Pat. No. 4,975,326 to Buylous, U.S. Pat. No. 4,355,132 to East, U.S. Pat. No. 4,414,169 to McClary, and RE 36,698 to Kim). However, all or almost all past attempts have focused on yarns with a DPF of lower than 5 since it was generally believed that increasing DPF decreases fatigue resistance (see e.g., Bailievier U.S. Pat. No. 5,285,623). Furthermore, it is believed that in many yarns fatigue strength retention tends to decrease or remain substantially the same as the filament count increases.
Also, PET treated cords have been produced using Hoechst T748 with a DPF of 7.2, which exhibited similar fatigue resistance when compared to treated cords from a 4.8 DPF yarn. Thus, there is still a need to provide compositions and methods for production of dimensionally stable yarns with improved fatigue strength retention characteristics.